Our laboratory continues the development of a comprehensive model of DNA radiation induced to damage, one that describes physicochemical events from the initial track structure and DNA ion-radical-excited state formation through hole and electron transfer, to chemical events involving free radical processes that lead to secondary radicals and, finally, to combination and redox processes that result in DNA damage such as base and sugar damage, strand scission and associated base release. The spatial distribution of the damage sites is a critical portion of our model as it determines the biological effectivenes of the radiation damage. The overall goal of the current proposed effort is to test several aspects this overall model, to modify it as appropriate and thereby elucidate fundamental mechanisms of radiation damage to DNA by radiations of varying linear energy transfer (LET). These studies will be performed under conditions that emphasize the direct effect of radiation, and will employ magnetic resonance spectroscopies (ESR), product analyses, gamma and cyclotron heavy ion-beam irradiation, as well as theoretical modeling including time dependent density functional theory (TD- DFT). The first aim will test the hypotheses that the mechanism of hole transfer process from the sugar- phosphate backbone to the DNA bases is base sequence and temperature dependent. Oligomer sequences chosen for study will provide the sequence dependence and temperature dependence of the mechanism of long range hole transfer from the DNA backbone through A runs to a remote G in DNA. The second aim will test the hypothesis that the yields of two strand break radicals, at C5' and C3' sites in the DNA sugar phosphate backbone are LET dependent and are produced by specific oxidative and reductive mechanisms, respectively. We will test the hypothesis that, as LET increases, spatial clustering of these radicals increases to form multiple proximate strand breaks (multiple damage sites); however, the highest yields of sugar radicals do not occur precisely at the Bragg peak of the ion beam path. This would suggest that ion and radical recombinations inhibits cluster formation at the track ends. The third aim will employ theoretical calculations to further test and confirm molecular mechanisms proposed in each of the aims. Especially significant will be the use of DFT theory to accurately predict core-excited states and test the hypotheses that one low energy electron can induce a double strand break and further that double oxidation of sugar sites leads to strand breaks without radical involvement. We believe these efforts will allow us to establish new insights into fundamental radiation processes important for biomedical research.